Process and apparatus for producing ammonium sulfate crystals

Abstract

The present invention provides a continuous process for producing ammonium sulfate crystals, wherein said process comprises: (a) feeding to a first group of crystallization sections, which crystallization sections are heat integrated in series, a first aqueous ammonium sulfate solution that contains one or more impurities; (b) feeding to a second group of crystallization sections, which crystallization sections are heat integrated in series, a second aqueous ammonium sulfate solution that contains one or more impurities; (c) crystallizing ammonium sulfate crystals in each crystallization section respectively from each of said solutions of ammonium sulfate that contain one or more impurities; (d) purging a fraction of the ammonium sulfate solution that contains one or more impurities from each of said crystallization sections; and (e) discharging ammonium sulfate crystals from each crystallization section, characterized in that: (i) both the first group of crystallization sections and the second group of crystallization sections are together heat integrated in one series of crystallization sections; wherein the first group of crystallization sections operates at higher temperature than the second group of crystallization sections; and (ii) the composition of the first aqueous ammonium sulfate solution that contains one or more impurities is different to the composition of the second aqueous ammonium sulfate solution that contains one or more impurities. Further provided is apparatus suitable for producing ammonium sulfate crystals.

Claims

1. A continuous process for producing ammonium sulfate crystals, wherein said process comprises: (a) feeding to a first group of crystallization sections, which crystallization sections are heat integrated in series, a first aqueous ammonium sulfate solution that contains one or more impurities; (b) feeding to a second group of crystallization sections, which crystallization sections are heat integrated in series, a second aqueous ammonium sulfate solution that contains one or more impurities; (c) crystallizing ammonium sulfate crystals in each crystallization section respectively from each of said solutions of ammonium sulfate that contain one or more impurities; (d) purging a fraction of the ammonium sulfate solution that contains one or more impurities from each of said crystallization sections; and (e) discharging ammonium sulfate crystals from each crystallization section, characterized in that: (i) both the first group of crystallization sections and the second group of crystallization sections are together heat integrated in one series of crystallization sections; wherein the first group of crystallization sections operates at higher temperature than the second group of crystallization sections; and (ii) the composition of the first aqueous ammonium sulfate solution that contains one or more impurities is different to the composition of the second aqueous ammonium sulfate solution that contains one or more impurities; and (iii) the first aqueous ammonium sulfate solution that contains one or more impurities and the second aqueous ammonium sulfate solution that contains one or more impurities are each independently produced as by-products during the production of cyclohexanone oxime, caprolactam, or acrylonitrile.

2. A process according to claim 1, wherein the crystallization sections are heat integrated by means of steam.

3. A process according to claim 2, wherein the temperature of steam entering the first crystallization section in the series of crystallization sections is from 80 C. to 160 C.

4. A process according to claim 3, wherein the temperature of steam exiting the last crystallization section in the series of crystallization sections is from 45 C. to 75 C.

5. A process according to claim 1, wherein the first aqueous ammonium sulfate solution that contains one or more impurities is produced as a by-product during the production of cyclohexanone oxime by oximation of cyclohexanone with aqueous hydroxylammonium sulfate.

6. A process according to claim 1, wherein the second aqueous ammonium sulfate solution that contains one or more impurities is produced as a by-product during the production of caprolactam obtained by Beckmann rearrangement of cyclohexanone oxime in either oleum, sulfuric acid, or SO.sub.3.

7. A process according to claim 1, wherein the series of crystallization sections comprises from 2 to 5 crystallization sections.

8. A process according to claim 1, wherein each crystallization section in the series of crystallization sections has substantially equal production capacity of ammonium sulfate crystals.

9. A process according to claim 1, wherein a fraction of aqueous ammonium sulfate solution that also contains one or more impurities is purged from at least one crystallization section in a group to at least one other crystallization section in the same group.

10. A process according to claim 9, wherein the crystallization sections are heat integrated by means of steam, and a fraction of aqueous ammonium sulfate solution that also contains one or more impurities is purged from each crystallization section in a group to the next crystallization section, as defined by descending temperature of steam supply, in the same group, with the exception that the purge from the final crystallization section in the group is discharged from the group.

Description

(1) The present invention will be more fully explained with reference to the following drawings.

(2) FIG. 1 describes an embodiment of the prior art, wherein four crystallization sections are arranged in parallel in view of the feed of ammonium sulfate solution.

(3) FIG. 2 describes an embodiment of the present invention, wherein the common feed line is adapted to enable feeding solutions of ammonium sulfate with different compositions to two groups of two crystallization sections.

(4) FIG. 3 describes an embodiment of the present invention comprising two parallel series, each of four crystallization sections, each series heat integrated in effect. Two feed ammonium sulfate solutions are fed to four groups of crystallization sections across the two series.

(5) FIG. 1 describes an embodiment of the prior art. Four crystallization sections, (1), (2), (3), (4), each comprising a crystallizer of equal size are arranged in parallel with respect to the feed of ammonium sulfate solution. An ammonium sulfate solution passes through feed line (5) into each crystallization section, where crystallization occurs to form a slurry of ammonium sulfate crystals in an ammonium sulfate solution. The ammonium sulfate solution that passes through feed line (5) might originate from one single source or might have been obtained by blending two or more solutions of ammonium sulfate originating from different sources.

(6) Steam is fed to the crystallization section (1), via line (6), where it is used to evaporate solvent from the ammonium sulfate solution, thereby aiding crystallization. The steam does not directly contact the ammonium sulfate solution, but transfers heat indirectly thereto via a heat exchange unit. A solvent-comprising vapor stream is formed in crystallization section (1), and is discharged through line (7) to crystallization section (2), where it is used to evaporate solvent, analogous to the process in crystallization section (1). The solvent-comprising vapor stream formed in crystallization section (2) is discharged through line (8) to crystallization section (3) where it is used to evaporate solvent analogous to the process in crystallization section (1). The solvent-comprising vapor stream formed in crystallization section (3) is discharged through line (9) to crystallization section (4) where it is used to evaporate solvent analogous to the process in crystallization section (1). The solvent-comprising vapor stream formed in crystallization section (4) is discharged via line (10). Ammonium sulfate crystals are discharged from crystallization section (1) though line (11) for further processing. A fraction of ammonium sulfate solution comprising impurities is purged through line (12). Ammonium sulfate crystals are discharged from crystallization section (2) though line (13) for further processing. A fraction of ammonium sulfate solution comprising impurities is purged through line (14). Ammonium sulfate crystals are discharged from crystallization section (3) though line (15) for further processing. A fraction of ammonium sulfate solution comprising impurities is purged through line (16). Ammonium sulfate crystals are discharged from crystallization section (4) though line (17) for further processing. A fraction of ammonium sulfate solution comprising impurities is purged through line (18). Optionally, the ammonium sulfate crystals from lines (11), (13), (15) and (17) are combined, either before or after any further processing step. The solutions of ammonium sulfate purged through lines (12), (14), (16) and (18) are treated as waste, and undergo further processing. Optionally, these purged solutions of ammonium sulfate are fed to another crystallization section. Optionally, these solutions of ammonium sulfate are combined.

(7) FIG. 2 describes an embodiment of the present invention. The system is essentially the same as that of FIG. 1. Specifically, crystallization sections (1), (2), (3) and (4); the steam system (6), (7), (8), (9), (10); the four lines through which ammonium sulfate crystals are discharged from the crystallization sections (11), (13), (15), (17); and purge lines (12), (14), (16) and (18) are identical to those of FIG. 1.

(8) The feeds of solutions of ammonium sulfate to crystallization sections (1), (2), (3) and (4) are adapted. Instead of feeding a common aqueous ammonium sulfate solution to each of the crystallization sections (1), (2), (3) and (4), a first aqueous ammonium sulfate solution that contains one or more impurities is fed via line (5a) to a first group of crystallization sections, comprising (1) and (2); and a second aqueous ammonium sulfate solution that contains one or more impurities is fed via line (5b) to a second group of crystallization sections, comprising (3) and (4).

(9) FIG. 3 describes an embodiment of the present invention. The system is similar to that of FIG. 2 except that it comprises two parallel series of four crystallizers, each series being heat integrated. Specifically, crystallization sections (1), (2), (3) and (4); the steam system (6), (7), (8), (9), (10); the four lines through which ammonium sulfate crystals are discharged from these crystallization sections (11), (13), (15), (17); and purge lines (12), (14), (16) and (18) are identical to those of FIG. 2. A parallel series of crystallization sections, (1a), (2a), (3a) and (4a); steam system (6a), (7a), (8a), (9a), (10a); lines through which ammonium sulfate crystals are discharged from these crystallization sections (11a), (13a), (15a), (17a); and purge lines (12a), (14a), (16a) and (18a) are analogous to the first series of crystallization sections described with reference to FIG. 2. These correspond to the numbered components of FIG. 2 without the a.

(10) The feeds of aqueous ammonium sulfate solutions to the crystallization sections are adapted. To crystallization sections (1), (2), (1a), (2a) and (3a) a first aqueous ammonium sulfate solution is fed via line (5e). To crystallization sections (3), (4) and (4a) a second aqueous ammonium sulfate solution is fed via line (50. To each crystallization section roughly a similar amount of ammonium sulfate solution is fed. Accordingly, crystallization sections (1) and (2) form a first group; crystallization sections (3) and (4) form a second group; crystallization sections (1a), (2a) and (3a) form a third group; and crystallization section (4a) forms a fourth group.

(11) The invention is illustrated by but not intended to be limited to the following Examples.

EXAMPLE 1

(12) In a commercial caprolactam plant cyclohexanone oxime was produced according to the Raschig route from cyclohexanone produced via hydrogenation of phenol. The cyclohexanone oxime was converted into caprolactam in a multi-stage Beckmann rearrangement process with oleum. The obtained caprolactam was recovered after neutralization with aqueous ammonia. In each of the cyclohexanone oxime formation step and caprolactam formation step, aqueous ammonia was used for neutralization. As a result an aqueous ammonium sulfate solution was obtained as by-product in each step.

(13) The composition of the aqueous ammonium sulfate solution obtained in the cyclohexanone oxime formation step was:

(14) TABLE-US-00001 Ammonium sulfate ca. 43.5 wt. % Water ca. 54.4 wt. % Free H.sub.2SO.sub.4 <0.1 wt. % COD ca. 120 ppm Ammonium nitrate ca. 2.1 wt. %

(15) The composition of the aqueous ammonium sulfate solution obtained in the caprolactam formation step was:

(16) TABLE-US-00002 Ammonium sulfate ca. 44 wt. % Water ca. 56 wt. % Free H.sub.2SO.sub.4 <0.1 wt. % COD 1200 ppm Ammonium nitrate <0.01 wt. %

(17) COD (chemical oxygen demand) content, which is a measure for the concentration organic impurities, refers to values as determined according to ASTM D 1252-95 (dichromate method).

(18) The volume:volume ratio of aqueous ammonium sulfate solution obtained in the cyclohexanone oxime formation step to the aqueous ammonium sulfate solution obtained in the caprolactam formation step was approximately 5:3.

(19) By addition of aqueous ammonia (about 25 wt. %) the pH value of both ammonium sulfate solutions were increased to about 5 (as determined at a temperature of 25 C.).

(20) The resulting solutions were fed to two lines of each four crystallization sections, in a system depicted in FIG. 3.

(21) To crystallization sections (1), (2), (1a), (2a) and (3a) the pH adjusted aqueous ammonium sulfate solution obtained in the cyclohexanone oxime formation step was fed via line (5e). To crystallization sections (3), (4) and (4a) the pH adjusted aqueous ammonium sulfate solution obtained in the caprolactam formation step was fed via line (5f). To each crystallization section roughly a similar amount of ammonium sulfate solution was fed.

(22) The crystallizers in the crystallization sections (1) and (1a) were operated at a temperature of about 115 C. The crystallizers in the crystallization sections (2) and (2a) were operated at a temperature of about 90 C. The crystallizers in the crystallization sections (3) and (3a) were operated at a temperature of about 70 C. And those in the sections (4) and (4a) were operated at a temperature of about 50 C. All crystallizers were of the Oslo crystallizer type.

(23) The amount of fresh steam that were fed via lines (6) and (6a) to the crystallization sections (1) and (1a) was in each case about 10 ton/hr.

(24) By purging aqueous ammonium sulfate solution, COD levels in the crystallization sections (4), (3a) and (4a) were kept at levels of approximately 40, 30 and 40 gram per kg clear solution, respectively. By purging aqueous ammonium sulfate solution, the ammonium nitrate levels in clear solution in the crystallization sections (1), (2), (1a), (2a) and (3a) were kept at levels of approximately 35 wt. %. From each crystallization section, ammonium sulfate solution containing ammonium sulfate crystals was discharged and fed to a centrifuge in which the crystals were separated from mother liquor and were washed with some water. Then the obtained washed crystals were dried.

(25) The colour of the resulting ammonium sulfate crystals was white and no black coloured particles were observed between the salt crystals.

(26) The production capacity of ammonium sulfate crystals of each crystallization section was about 60 kta.

(27) This example shows that by feeding an aqueous ammonium sulfate solution obtained in the cyclohexanone oxime formation step to the crystallization sections that are operated at higher temperatures and feeding an aqueous ammonium sulfate solution obtained in the caprolactam formation step it is possible to produce ammonium sulfate crystals that are not polluted with black coloured particles.

(28) The combined amount of fresh steam that was fed via lines (6) and (6a) to the crystallization sections (1) and (1a) was about 20 ton/hr, In case both aqueous ammonium sulfate solutions would have been fed to 8 crystallization sections without heat integration the total consumption of fresh steam would have been for each section about 10 ton/hr. So, this example further shows that steam (energy) consumption may be significantly reduced; in theory by 75%.

COMPARATIVE EXAMPLE 1

(29) In a commercial caprolactam plant cyclohexanone oxime was produced according to the Raschig route from cyclohexanone produced via hydrogenation of phenol. The cyclohexanone oxime was converted into caprolactam in a multi-stage Beckmann rearrangement process with oleum. The obtained caprolactam was recovered after neutralization with aqueous ammonia. In each of the cyclohexanone oxime formation step and in the caprolactam formation step aqueous ammonia was used for neutralization and as a result an aqueous ammonium sulfate solution was obtained as by-product.

(30) The volume:volume ratio of the amount of aqueous ammonium sulfate solution obtained in the cyclohexanone oxime formation step to the amount of aqueous ammonium sulfate solution obtained in the caprolactam formation step was approximately 5:3. These two aqueous ammonium sulfate solutions were blended.

(31) The composition of the combined aqueous ammonium sulfate solutions was:

(32) TABLE-US-00003 Ammonium sulfate ca. 43.7 wt. % Water ca. 55 wt. % Free H.sub.2SO.sub.4 <0.1 wt. % COD ca. 525 ppm Ammonium nitrate ca. 1.3 wt. %

(33) By addition of aqueous ammonia (about 25 wt. %) the pH value of combined ammonium sulfate solutions was increased to about 5 (as determined at a temperature of 25 C.).

(34) The obtained pH adjusted aqueous ammonium sulfate solution was fed to all four crystallization sections of an experimental set-up as described in FIG. 1. To each crystallization section roughly a similar amount of ammonium sulfate solution was fed.

(35) The temperatures of the crystallizers in the crystallization sections (1), (2), (3) and (4) were about 115 C., 90 C., 70 C. and 50 C., respectively.

(36) In order to obtain the same overall ammonium sulfate crystal yield as Example 1 the ratios of purge flow over feed for each crystallizer were taken equal to those in Example 1. Specifically, the ratio of purge flow over feed of crystallization section (1) was taken equal to the average of the ratios of purge flow over feed of crystallization sections (1) and (1a) in Example 1; the ratio of purge flow over feed of crystallization section (2) was taken equal to the average of the ratios of purge flow over feed of crystallization sections (2) and (2a) in Example 1; the ratio of purge flow over feed of crystallization section (3) was taken equal to the average of the ratios of purge flow over feed of crystallization sections (3) and (3a) in Example 1; and the ratio of purge flow over feed of crystallization section (4) was taken equal to the average of the ratios of purge flow over feed of crystallization sections (4) and (4a) in Example 1.

(37) From all crystallizers, the flows containing ammonium sulfate crystals were discharged and via centrifugation the crystals were separated from mother liquor and were washed with water. Then the obtained washed crystals were dried.

(38) The colour of the resulting ammonium sulfate crystals obtained from crystallization sections (1) and (2) was brownish and black coloured particles could be observed between the salt crystals. In the ammonium sulfate crystals obtained from crystallization sections (3) and (4) no black coloured particles were observed.

(39) This example shows that by feeding a blend of the aqueous ammonium sulfate solution obtained in the cyclohexanone oxime formation step and the aqueous ammonium sulfate solution obtained in the caprolactam formation step it is possible to produce ammonium sulfate crystals with the same overall ammonium sulfate crystal yield per tonne of produced ammonium sulfate crystals due to operating the crystallizers with the same purge to feed rates as Example 1. It is clear that after implementing this heat integration the same low overall consumption of heating steam per tonne of produced ammonium sulfate crystals can be obtained as Example 1.

(40) However, due to the poor quality of the ammonium sulfate crystals produced in the crystallization sections (1) and (2) the average quality of all ammonium sulfate crystals produced in Comparative Example 1 is much worse than the average quality of all ammonium sulfate crystals produced in Example 1.

COMPARATIVE EXAMPLE 2

(41) In this Comparative Example 2 the same blend of two aqueous ammonium sulfate solutions was used as in Comparative Example 1. By addition of aqueous ammonia (about 25 wt. %) the pH value of combined ammonium sulfate solutions was increased to about 5 (as determined at a temperature of 25 C.).

(42) The obtained pH adjusted aqueous ammonium sulfate solution was fed to the crystallization sections (3) and (4) of an experimental set-up as described in FIG. 1. The crystallization sections (1) and (2) of the experimental set-up as described in FIG. 1 were not in operation. Fresh steam was fed to the crystallization section (3) via line (8). The temperatures of the crystallizers in the sections (3) and (4) were taken equal to those in Comparative Example 1: about 70 C. and 50 C., respectively.

(43) The ratio of purge flow over feed of crystallization section (3) was taken equal to the ratio of purge flow over feed of crystallization section (3) in Comparative Example 1; and the ratio of purge flow over feed of crystallization section (4) was taken equal to the ratio of purge flow over feed of crystallization section (4) in Comparative Example 1.

(44) From all crystallizers ammonium sulfate solution containing ammonium sulfate crystals was discharged and via centrifugation the crystals were separated from mother liquor and were washed with water. Then the obtained washed crystals were dried.

(45) The resulting ammonium sulfate crystals obtained from crystallization sections (3) and (4) were white coloured and no black coloured particles were observed.

(46) This example shows that by feeding a blend of the aqueous ammonium sulfate solution obtained in the cyclohexanone oxime formation step and the aqueous ammonium sulfate solution obtained in the caprolactam formation step it is possible to produce good quality ammonium sulfate crystals (white coloured and without black particles).

(47) However, operating the evaporative crystallizers heat integrated in a series of just two instead of four results in an overall consumption of heating steam per tonne of produced ammonium sulfate crystals that is almost twice as high as the overall consumption of heating steam per tonne of produced ammonium sulfate crystals produced in Example 1.

COMPARATIVE EXAMPLE 3

(48) In a commercial caprolactam plant caprolactam is produced from cyclohexanone oxime via a 3-stage Beckmann rearrangement process in oleum. The obtained caprolactam was recovered after neutralization of the reaction mixture with aqueous ammonia. The resulting aqueous ammonium containing sulfate solution was extracted with benzene to recover caprolactam. After stripping, the resulting aqueous ammonium sulfate containing solution was sent to the crystallization section. Here the pH value of the stripped aqueous ammonium containing sulfate solution which had a temperature of about 60 C. was adjusted by adding aqueous ammonia to a value of about 5 (as determined at a temperature of 25 C.). The resulting solution was fed to an Oslo type crystallizer that was operated at a temperature of about 115 C. The annual capacity of this Oslo crystallizer was about 75 kton ammonium sulfate crystals. By purging ammonium sulfate solution, the COD level in the crystallizer was kept at a level of approximately 15 gram per kg clear solution. Ammonium sulfate solution containing ammonium sulfate crystals was discharged from this crystallizer and fed to a centrifuge in which the crystals were separated from the mother liquor and washed with water. Then the obtained washed crystals were dried.

(49) The colour of the resulting ammonium sulfate crystals was brownish, and black coloured particles that were irregularly shaped and of sizes up to a few millimeters could be observed between the salt crystals.

(50) Four of these black coloured particles were hand-picked and analysed. The results of these analyses are shown in the next Table:

(51) TABLE-US-00004 Component Particle 1 Particle 2 Particle 3 Particle 4 Water 12.3 wt. % 30.4 wt. % 1.4 wt. % 5.8 wt. % Ammonia 14.5 wt. % 6.0 wt. % 9.7 wt. % 11.0 wt. % Sulfate 35.0 wt. % 8.6 wt. % 17.6 wt. % 21.2 wt. % Caprolactam 0.16 wt. % 0.14 wt. % 0.16 wt. % 0.5 wt. % -aminocaproic 0.28 wt. % 0.22 wt. % 0.43 wt. % 0.43 wt. % acid Disulfonated 9.9 wt. % 15.4 wt. % 19.8 wt. % 20.2 wt. % octahydro- phenazine Others Balance Balance Balance Balance Non-aqueous 3.2 wt. % 4.4 wt. % 6.9 wt. % 7.6 wt. % soluble residue

(52) This Comparative Example 3 shows that undesired visible solid impurities are present when a feed of ammonium sulphate formed as by-product in the Beckmann rearrangement of cyclohexanone oxime to form caprolactam, when it is crystallized at a temperature of 115 C. Further, that these undesired visible solid impurities have a high organic content.